As the demand for clean energy sources increases, more research effort is being placed on fuel cells, wind-generation, and photovoltaic energy sources. Fuel cells are expected to be used in a wide variety of applications including stationary electric power plants, motive and auxiliary power for vehicles, and micro-power applications. Fuel cells are considered to be a major contribution to future energy generation devices due to energy efficiency and environmental friendliness.
For many stationary and portable power applications, the general structure is to have a low voltage fuel cell as the primary source, a DC/DC converter to obtain isolated high voltage, and a DC/AC inverter to obtain AC voltage. In a typical fuel cell power conversion system, the input source is low voltage DC and the output is high voltage AC. Solid oxide fuel cells (SOFC) are a promising option for high powered applications such as industrial applications and distributed electrical power generators. SOFC are constructed entirely from solid state materials, typically using a thin layer of stabilized zirconia (zirconium oxide) as the electrolyte in conjunction with a lanthanum manganate cathode and nickel zirconia anode.
Unfortunately, a fuel cell power system that contains a single phase DC/AC inverter tends to draw an AC ripple current at twice the output frequency. Such a ripple current may shorten fuel cell life span and worsen the fuel efficiency due to the hysteresis effect. The most obvious impact is the reduction of fuel cell output capacity because the fuel cell controller trips under instantaneous over current condition.
An example system, that has been the target development of the U.S. Department of Energy (DOE) Solid State Energy Conversion Alliance (SECA), is to have a nominal 5 kW single phase AC output for residential power system using the low voltage solid oxide fuel cell (SOFC), which has an output voltage ranging from 20 to 50V. In order to generate the 120/240V AC required for general home applications, a DC/DC converter is essential to boost fuel cell output voltage to a level that can be used by the inverter. A desired inverter DC input voltage for 120V AC generation is 200V, while 400V is desired for 240V AC generation. Any increase in efficiency is greatly desired. An increase in efficiency of only 1% is worth $75/kW given a $6.50/mbtu gas cost for a SOFC power plant the size of about 150 kW.
Some existing commercial off the shelf proton exchange membrane (PEM) fuel cells also have their nominal voltage set at 48V (and below) for either telecommunication or residential applications. In order for low voltage DC fuel cells to generate 50/60 Hz 120/240V AC for residential applications, a DC/DC converter is needed to boost the fuel cell voltage to a level that can be converted to the desired AC output.
Similarly, when a DC source is used for AC applications, such as Uninterrupted Power Supply (UPS) or distributed power, for standalone or grid tie applications, an inverter is needed to generate the required AC for the load. Using the power system of the United States of America as an example, the output load draws 60 Hz AC from the inverter. The AC needs to be rectified at the inverter input, and its frequency will be doubled; therefore, 120 Hz pulsating DC needs to be drawn from the DC/DC converter. The ripple component of this pulsating current will reflect back to the input side of the DC/DC converter, loading the DC power source such as a fuel cell with low frequency AC. This low frequency AC is referred to as a ripple current.
Additional issues are associated with a DC/DC converter, including cost, efficiency and reliability, as well as ripple current. Transient response along with auxiliary energy storage requirements are also a major consideration. Communication with a fuel cell controller and electromagnetic interference (EMI) emissions are of further concern.
Efforts have been extended to address these issues. Conventional passive methods with a single voltage loop controller may be used in an effort to dampen the ripple. Furthermore, adding an energy storage capacitor either on the high side of the DC bus to smooth the current at 200/400V level, or on the fuel cell bus to smooth ripple at a low voltage level, helps reduce the ripple. If the ripple current magnitude is 40% of the rated current, then the fuel cell source needs 20% additional power handling capability. Since the cost of the fuel cell is nontrivial, it is desirable to suppress ripple current to reduce the cost penalty.
Currently, the zero voltage switching (ZVS) pulse width modulation (PWM) full bridge converter is the dominate topology in high power DC/DC applications, although some research is being done on multi phase converters. Full bridge converters normally utilize large inductors to achieve ZVS, which will cause duty cycle loss and additional power loss during freewheeling. Metal oxide semiconductor field effect transistors (MOSFETs) are generally used as the switching device in a converter with low voltage and high current input. In such a converter, conduction loss introduced by the high circulating current may sacrifice the efficiency gained by soft switching. For a high power fuel cell system, high voltage conversion ratio and high input current are the major obstacles.
In order to provide a dual AC output, using an example specification with a nominal fuel cell having a DC output voltage of 22V and an AC load of 120/240V at 5 kW continuous and 10 kW peak, an isolated DC/DC converter is needed to convert low voltage DC to a DC voltage higher than 400V, sufficient for a 240V AC output. A DC/DC converter inevitably sees more than 240 A on the fuel cell side, making the design of a DC/DC converter with low voltage and high current desirable.
Desirable converters need to be capable of high power operation with a high voltage conversion ratio. A transformer is needed for both voltage boost and isolation; however, a high turns ratio is not favored due to potentially high leakage inductances. Furthermore, a high switching frequency is preferred to reduce the passive component size. In order to achieve a high switching frequency while improving converter efficiency, soft switching is necessary.
Among the soft switching techniques suitable for high power converter applications, phase shift (PS) control has been the favorite. However, for a single phase full bridge phase shift converter, the ZVS is achieved over a limited load range. Past efforts have focused on solving this problem. The most popular solutions are to add a saturable core or make some devices switch under zero current switching (ZCS) condition with added auxiliary circuitry.
Fuel cell current ripple reduction is a major issue for fuel cell converter design. It has been suggested that the ripple current be limited to less than 10%. Passive energy storage compensation methods have been suggested and tested extensively. Active compensation methods, with external bidirectional DC/DC converters, have been suggested. These methods require externally added components or circuits.
In conventional devices, losses occur during switch conduction, diode conduction, transformer, output rectifier, output filter inductor, output capacitor, input capacitor and parasitics, such as copper traces and other interconnects.